Twisted pair communication system, apparatus and method thereof

ABSTRACT

A communication system comprises a twisted pair communication link operably coupled to at least two driver stages for providing at least two independent input signals on the twisted pair communication link. The at least two independent input signals on the twisted pair communication link are summed and input to a comparator arranged to compare the summed signal to a reference value. The output of the comparator is input to the at least two driver stages. The outputs from the at least two driver stages are summed and fed back and summed with one or more of the independent input signals. 
     In this manner, adverse effects due to non-ideal symmetry between components in a twisted pair communication link, such as a Controller Area Network system, are reduced.

FIELD OF THE INVENTION

The preferred embodiment of the present invention relates to reductionof emission level in a twisted pair communication system. The inventionis applicable to, but not limited to, provision of a high speed two-wireCAN network.

BACKGROUND OF THE INVENTION

In the field of vehicle electronics, the use of a high speed controllerarea network (CAN) is becoming ever more prevalent. In a typicalautomotive application, the CAN provides a two-wire multiplexcommunication link that can be routed around the vehicle. Thus, the CANprovides a simple mechanism for a vehicle processing unit to communicateeffectively with remote electrical/signal processing units, e.g. vehiclelight modules, braking system, airbag modules, etc.

The CAN specifications for road vehicles are defined by theInternational Standards Organisation (ISO), as described below.

-   -   (i) ISO 11898-2: high speed physical layer—part 2;    -   (ii) ISO 11898-3: low speed fault tolerant physical layer—part        3;    -   (iii) ISO 11898-4: time trigger CAN; and    -   (iv) ISO 11898-5: high speed physical layer with low power mode        and wake up.

The CAN specifications support communication over, say, a ten metrelength. However, in requiring the CAN bus to support communication overthis length, the long wires act as an antenna and as such areeffectively subject to automotive electrical transients, as well asindustrial transients, such as electro-magnetic interference (EMI) andelectro-static discharge (ESD).

Furthermore, as the wires can be very long, the control of the slew rateof signals routed by the wires is also known to be very critical toavoid any EMC emission. In addition, in order to operate the CAN bussuccessfully, in a problematic vehicle environment; a CAN transceivermust also be able to withstand high voltage transients.

Consequently, it is important to guarantee a good control and matchingof the respective devices between the switching of the two CAN wires. Toguarantee good matching of the slew rate between the respective wires,the two wires (i.e. the high-side CAN (CAN-H) and the low-side CAN(CAN-L)) must be matched in performance and controlled equally.

A typical CAN driver circuit is illustrated in FIG. 1. The CAN drivercircuit comprises a digital transmit input signal 102 that is input toboth the CAN-H driver 104 and a CAN-L driver 106. In order to achieveboth high-speed and symmetry of operation, low-voltage matchedcomponents are generally used. The CAN-H driver utilizes, say, a pnptransistor 130 as an active device operably coupled to Vcc 108, where asthe CAN-L driver utilizes, say, a npn transistor 132 as an activedevice, operably coupled to ground 110.

The respective outputs 118, 120 from the CAN-H and CAN-L drivers 104,106 are input 114, 116 to a comparator 112. The output from thecomparator 112 is a ‘receive’ digital output signal 122. Thus, as willbe appreciated by a skilled artisan, the driver circuits that controlthe signals on the CAN-H wire 118 and CAN-L wire 120 need to becarefully matched, to ensure that the CAN-H driver 104 and CAN-L driver106 are adjusted to switch between the CAN-H and CAN-L wires 118, 120 inphase. A key aspect to using such a twisted cable is to ensure thecurrent contained in the wires (in both directions) is of equalamplitude and of opposite sign. In this manner, the magnetic fieldproduced, which is proportional to the current, is substantially zero.Thus, electro-magnetic interference due to the current in the twistedpair of cables is minimized. Clearly, any asymmetry between the currentin the two wires produces a magnetic field, which is highly undesirable.

In addition, it is known that the common-mode CAN-H bus wire and CAN-Lbus wire must also be constant during the switching transition, i.e.when a signal appears on the CAN-H and CAN-L wires, and when it is takenoff. Thus, the ΔI needs to be minimized during the transitions otherwiseelectromagnetic interference is created. In effect, there are two typesof common-mode configuration:

-   -   (i) Current: where the current value of both the CAN-H and CAN-L        wires should be equal but of opposite sign; and    -   (ii) Voltage: which should be equal, given that the for high        speed operation the bus impedance is specified as 60 ohms in the        CAN standard.

Furthermore, to avoid EMC emission, the slew rate applied to signals onthe CAN-H bus wire 118 and the CAN-L bus wire 120 must be controlled andmatched. The slew rate is a function of the temperature (delay) and ofthe load. Consequently, the slew rate is difficult to controlaccurately.

A graphical example of how difficult it is to achieve a good matchbetween the high-side driver and the low-side driver is shown in FIG. 2.Here, the transmit waveform signal 202 is shown with a slight offset tothe receive waveform signal 222. If the respective driver circuits aresymmetrical, the CAN-H and CAN-L signals in waveform 218 are alsosymmetrical, resulting in the summation of CAN-H (nominally 3.5V) andCAN-L (nominally 1.5V) values to be flat, at approximately 5V. Thesummation of the CAN-H and CAN-L signals is often referred to as the‘common mode’. However, when the driver circuits are not completelymatched, the symmetry between CAN-H and CAN-L during transitions is notmet. This lack of matching results in a so-called common-mode glitch 230(say a variation of the order of 120 mV), which is noticeable upon thesummation of the CAN-H and CAN-L values.

A solution to this problem is illustrated in the known prior art circuit300 of FIG. 3, with the use of multiple drivers 316, 318, 320, 322 forthe CAN-H and drivers 324, 326, 328 and 330 for CAN-L. A series of veryfast switches 336, 338, 340, 342, 344, 346, 348 and 350 are operablycoupled to respective serial resistances, where each driver operation iscontrolled by a fixed delay 304, 306, 308, 310, 312 and 314.

In this regard, the slew rate is fixed by the delay elements 304, 306,308, 310, 312 and 314, the series resistances and the load capacitance.Accurate selection of these components ensures a good match.

However, with a high voltage range on the output, non symmetricalclamping is (due to the inherent nature of the components) introduced inseries with resistance. These high voltage components can be designed tobe somewhat symmetrical at low frequencies. However, they will exhibitasymmetry at higher frequencies, say above 100 KHz. Thus, asymmetry ofsignals between the two wires will generate common-mode glitches at afrequency of above 100 KHz. In effect, a new common-mode (i.e. thesummation of the values of the CAN-H and CAN-L wires) exists at eachfrequency of operation.

European Patent EP 0955750, titled “Line driver with parallel driverstages” by Texas Instruments, as well as European Patent EP0763917titled “Line driver with pulse shaper” by Lucent Technologies Inc. U.S.Pat. No. 5,194,761 illustrates prior art CAN arrangements.

However, it is recognised that, in order to sustain higher and highervoltage levels during fault conditions, it is no longer possible to uselow voltage components. Thus, instead, high voltage components arerequired to be used, particularly components that exhibit higherparasitic effects. Notably, and problematically, the parasiticcomponents exhibit different characteristics for the high side driver(CAN-H) and the low side driver (CAN-L) outputs. Consequently, thecommon-mode performance becomes degraded with those high voltagecomponents.

Thus, a need exists for an improved twisted-pair based communicationsystem, apparatus and method therefor, particularly to drive CAN-H andCAN-L bus wires.

STATEMENT OF INVENTION

In accordance with preferred aspects of the present invention, there isprovided a communication system, an apparatus and method therefor toreduce emission levels due to non-ideal symmetry between components intwisted pair paths, as defined in the appended Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a known CAN driver circuit;

FIG. 2 shows a graphical example illustrating the difficulty inachieving a good match between a CAN-H driver and a CAN-L driver is aCAN bus system; and

FIG. 3 illustrates a known multiple CAN driver circuit that aims toprovide a good match between a CAN-H driver and a CAN-L driver.

Exemplary embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 4 illustrates a CAN driver circuit in accordance with the preferredembodiment of the present invention; and

FIG. 5 illustrates graphically how the circuit of FIG. 4 provides areduction in the common-mode error in accordance with the preferredembodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 4, a CAN driver circuit is illustrated inaccordance with the preferred embodiment of the present invention.Notably, the CAN-H signal 406 and the CAN-L signal 402 are input to asummation function 408. It is envisaged that the summation function maybe implemented in a variety of ways, such as a summing block, discretelogic gates, AC coupling capacitors, etc. The CAN-H signal 406 and theCAN-L signal 402 are thus summed and input 414 to a fast comparator 418at, say, a negative input port 414.

A reference voltage level 412 is applied to the positive input port 416of the comparator. The comparator 418 compares the summation of CAN-Hand CAN-L signals with a reference voltage 412, which in the preferredembodiment is set at 5V. The comparator 418 outputs the comparison toboth a CAN-H driver stage 424 and a CAN-L driver stage pair 422, 424.The CAN-H driver stage 424 is coupled to a supply voltage 426 and theCAN-L driver stage 422 is coupled to ground 428.

Although the preferred embodiment of the present invention is describedwith respect to using NMOS and PMOS transistors 422, 424 at the outputof the comparator 418 to generate a CAN-L feedback signal, it isenvisaged that these devices may be replaced by equivalent functions,such as switches or current sources or current sources with resistorscoupled thereto, etc.

Notably, the output from both the CAN-H driver stage 424 and the CAN-Ldriver stage 422 is summed and provided as a feedback signal to eitherthe CAN-H or CAN-L (or indeed both) signal(s). In comparing thesummation of ‘CAN-H+CAN-L’ to a reference voltage applied to thecomparator, the output of the comparator flips as soon as the summationgoes above or below the reference value. Thus, dependent upon thecomparison of the common-mode (summation of CAN-H and CAN-L, nominally5V) value to the reference value (of 5V), current is either drawn fromthe CAN-L or pushed into the CAN-L feedback path to compensate for thedifference. In this manner, the value of ‘CAN-H+CAN-L’ is eitherdecreased or increased via the feedback path.

The speed of the comparator defines the reaction time of this loop, andthus the difference between the summation of ‘CAN-H+CAN-L’ and thereference voltage.

Thus, in this manner, a mechanism to improve the performance of the CANcircuit 400 is described by summing the CAN-H signal 406 and CAN-Lsignal 402 and using a fast, continuously-operating comparator 418, e.g.one that provides sufficient speed and low offset between the two wires.Thereafter, the output from the comparator 418 is fed back to one orboth of the CAN signals 406, 402. A typical (CMOS) comparator error of10 mV will result in a common-mode glitch of 10 mV.

Furthermore, in this implementation, the current compensation is limitedto, say, 10% of the total current, to avoid an adverse affect on theperformance (e.g. slew rate, propagation time, etc.) when thecompensation system is disturbed by EMC perturbation (BCI, DPI, etc.).

In a CAN system, the transition rise time and fall time of the CAN-H andCAN-L signals has to be typically between 15 nsec and 35 nsec at a speedof 1M baud. Thus, it is envisaged that if the two CAN wires cannot beaccurately matched using a single driver stage, a series of respectivedriver stages may be used, where the series of small stages are locatedin parallel, as per the known system in FIG. 3.

In this regard, several stages may be sequentially triggered inparallel. For example, if 10 stages are located in series, for each ofthe parallel CAN-H and CAN-L paths, the comparator may need to switchbetween the stages every 1.5 nsec to 3.5 nsec per transition. Hence, inthis regard, a switching performance of the comparator of between 1 nsecand 5 nsec is likely to be acceptable.

Although, the preferred embodiment of the present invention is describedwith reference to a continuously-operating comparator, it is envisagedthat the inventive concept is equally applicable to a ‘time-discrete’comparator, as would be understood by a skilled artisan. In thiscontext, a ‘time-discrete’ comparator would encompass a ‘sample & hold’type of comparator, whereby values of the CAN-H and CAN-L signals aresampled and held and then compared in a time-discrete manner.Advantageously, a number of ‘sample & hold’ comparators offer anoffset-cancellation technique. The use of an offset-cancellationtechnique allows the removal of any common-mode glitch that occurs dueto the comparator offset.

It is also envisaged that known cancellation techniques may be appliedto continuously-operating comparators such that they also can implementcancellations of common-mode glitches due to a comparator offset.

In an enhanced embodiment of the present invention, it is envisaged thatthe common-mode glitch problem is only applied during transition stagesof the CAN system. In this regard, it is envisaged that the feedbackcircuitry is enabled at the start of a transition and disables after thetransition has ended. Advantageously, employing such a time-multiplexedarrangement assists in minimising disturbances in the CAN system causedby the aforementioned circuitry.

The use of a fast comparator in this manner compensates for anyreal-time imbalance between the signal values on the CAN-H and CAN-Lwires, thereby facilitating a dramatic reduction in emission level bylimiting the common-mode glitch that occurs during transition on the buslines.

Advantageously, the current output capability of the CAN compensationcircuit is limited to a certain (low) value, in contrast to the mainCAN-L and CAN-H capabilities. Indeed the slew rate on (CAN-H or CAN-L)is directly proportional to the output current. Thus, by using a seriesof smaller MOSFETs, any variation of current in, say, the CAN-L linecauses minimal modification to the slew rate. Consequently, it is anadvantage to limit the current transition, particularly in vehicularapplications, to minimise the effect of radiated emissions due to, say,mobile antenna(s) or portable TVs.

At each stage transition, the comparator 418 thus compensates theintrinsic error of common-mode ‘CAN-H+CAN-L’ by continuously adding orsubtracting current 430 in order to keep a common-mode valuesubstantially equal to the reference value 412. By ensuring that thesummation value of ‘CAN-H+CAN-L’ is made substantially equal to apredefined reference value, any electromagnetic emission resulting fromusing a twisted pair of wires (as per the CAN system) is effectivelyreduced, i.e. the magnetic fields generated by each wire are opposed insign and therefore the resultant value of the summation is substantially‘zero’.

Referring now to FIG. 5, a graph illustrates how the circuit of FIG. 4provides a reduction in the common-mode error in accordance with thepreferred embodiment of the present invention. FIG. 5 illustrates atransition between CAN-H and CAN-L signals, and in particular thecorrection current that maintains a low glitch level.

Although the preferred embodiment of the present invention has beendescribed with reference to a CAN circuit, it is envisaged that, foralternative applications, the inventive concept may be applied to anycommunication system that employs twisted pair cabling, such as thoseused by Ethernet transceivers.

It will be understood that the communication system, apparatus andmethod for reducing emission levels in a two-wire communication system,such as a CAN system as described above, aims to provide at least one ormore of the following advantages:

-   -   (i) Improve the non-ideal symmetry on CAN-H, CAN-L drivers;    -   (ii) Reduce the reliance on matched components within the device        used between the two wires;    -   (iii) The common-mode glitch problem associated with bus        transitions is better controlled;    -   (iv) The preferred embodiment is very robust to process        variations, due to the introduction of the feedback loop;    -   (v) The EMC emission is dramatically reduced; and    -   (vi) The circuit of the preferred embodiment is such that it is        very easy to reuse on other applications/technologies; i.e.        substantially no software/hardware modifications are required.

In particular, it is envisaged that the aforementioned inventive conceptcan be applied by a semiconductor manufacturer to any device orintegrated circuit for use in any communication system employing twistedcabling. It is further envisaged that, for example, a semiconductormanufacturer may employ the inventive concept in a design of astand-alone device, such as an Ethernet transceiver, or an embeddedmodule in an application-specific integrated circuit (ASIC) and/or anyother sub-system element.

Whilst the specific and preferred implementations of the embodiments ofthe present invention are described above, it is clear that one skilledin the art could readily apply variations and modifications of such aninventive concept.

Thus, an improved twisted pair communication system, apparatus andmethod therefor; to reduce emission levels due to non-ideal symmetrybetween components in the respective twisted pair paths have beendescribed, wherein the aforementioned disadvantages with prior artarrangements have been substantially alleviated.

1. A communication system comprising: a twisted pair communication linkoperably coupled to at least two driver stages for providing at leasttwo independent input signals on the twisted pair communication link,wherein the at least two independent input signals on the twisted paircommunication link are summed and input to a comparator arranged tocompare the summed signal to a constant reference value, wherein anoutput of the comparator is input to the at least two driver stages andoutputs from the at least two driver stages are summed together, fedback, and summed with one or more of the at least two independent inputsignals.
 2. A communication system according to claim 1 wherein thecommunication system supports a Controller Area Network communication.3. A communication system according to claim 1 wherein the comparatorand the at least two driver stages are arranged to compensate forcommon-mode error by adding or subtracting current to be summed with theone or more of the independent input signals to substantially track thereference value.
 4. A communication system according to claim 1, whereinthe comparator is fast continuously-operating comparator or a‘time-discrete’ comparator.
 5. A communication system according to claim1, wherein the communication system encompasses a plurality of outputstages of the twisted pair communication link arranged in parallel toproduce one of a CAN-H output and/or one CAN-L output.
 6. Acommunication system according to claim 5 wherein the plurality ofstages are turned on in sequence.
 7. Apparatus for a communicationcircuit comprising: a twisted pair communication link operably coupledto at least two driver stages for providing at least two independentinput signals on the twisted pair communication link, wherein the atleast two independent signals on the twisted pair communication link aresummed and input to a comparator that compares the summed signal to aconstant reference value, wherein an output of the comparator is inputto the at least two driver stages and outputs from the at least twodriver stages are summed, fed back, and summed with one or more of theat least two independent input signals.
 8. Apparatus for a communicationcircuit according to claim 7 wherein the apparatus supports ControllerArea Network (CAN) communication.
 9. Apparatus according to claim 7wherein the comparator and the at least two driver stages are arrangedto compensate for common-mode error by adding or subtracting current tobe summed with the one or more of the independent input signals tosubstantially track the reference value.
 10. Apparatus according toclaim 7 wherein the comparator is fast continuously-operating comparatoror a ‘time-discrete’ comparator.
 11. Apparatus according to claim 7wherein the communication system encompasses a plurality of outputstages of the twisted pair communication link arranged in parallel toproduce one of a CAN-H output and/or one CAN-L output.
 12. Apparatusaccording to claim 11 wherein the plurality of stages are turned on insequence.
 13. A method of reducing common-mode error in a communicationsystem comprising a twisted pair communication link comprising the stepsof: providing at least two independent input signals on the twisted paircommunication link; summing the at least two independent input signalson the twisted pair communication link; comparing at a comparator thesummed signal to a constant reference value; inputting an output of thecomparator to at least two driver stages; summing the outputs from theat least two driver stages; feeding back the summed output from the atleast two driver stages to one or more of the at least two independentinput signals; and summing the feedback summed output with the one ormore of the at least two independent input signals.